12. Interval between ASDIC acquisition if DC's are dropped.
I can't provide any evidence for this, and I'm no expert, but from what I've read early decoys operated by making clouds of bubbles when they reacted with sea-water, with this design arising because it was already clear to both sides that ASDIC produced very muddled return signals, or failed to work at all, if the water-column above the boat was aerated by the DC explosion. This is useful, as this feature could be used to help make DC attacks more prolonged but less lethal, especially in shallower-water.
13. Salinity. I'd be interested to know if U-boats possessed salinity gauges? If so, changing salinity values can add some much needed workload to the Dive Officer. This could be added even if no gauges were fitted, as just something to increase DO workload.
14. Thermal layers. These bands of water at different temperatures can cause ASDIC to be reduced in effectiveness. Certainly US submarines of the period had gauges for outside water temperature.
15. "Soaping" depth charges.
I forget the range bands of standard British (Or US) depth-charges were, but at least early in the war, they could not reach the depths U-boats could get down to. Standard practice when a U-boat sank to beyond the nominal depth setting was to stuff the hole through which water pressure (and therefore depth) was admitted with normal soap. This dissolved as it sank, allowing the DC to explode lower than would otherwise be the case. If this were featured, and the depth attained by any DC be a random amount beyond the nominal set-value, then it would allow an escort to remain something of a threat and suppress the u-boat even if he can't officially reach the correct depth. At some point, probably circa 1942, the Royal Navy DC's were given deeper possible settings to clobber those "hard to reach" u-boats! I also suggest that manned ASDIC's on escorts be allowed, if no thermal layer intervening, to successfully ping off a u-boat at 185m or more.
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